Extracting double-quantum coherence in two-dimensional electronic spectroscopy under pump-probe geometry
Two-dimensional electronic spectroscopy (2DES) can be implemented with different geometries, e.g., BOXCARS, collinear and pump-probe geometries. The pump-probe geometry has its advantage of overlapping only two beams and reducing phase cycling steps. However, its applications are typically limited to observe the dynamics with single-quantum coherence and population, leaving the challenge to measure the dynamics of the double-quantum (2Q) coherence, which reflects the many-body interactions. We propose an experimental technique in 2DES under pump-probe geometry with a designed pulse sequence and the signal processing method to extract 2Q coherence. In the designed pulse sequence with the probe pulse arriving earlier than pump pulses, our measured signal includes the 2Q signal as well as the zero-quantum (0Q) signal. With phase cycling and the data processing using causality enforcement, we extract the 2Q signal. The proposal is demonstrated with the rubidium atoms. And we observe the collective resonances of two-body dipole-dipole interactions of both $D_{1}$ and $D_{2}$ lines.